Pressure support system and method

ABSTRACT

A pressure generating system and method for generating a flow of fluid at a pressure above atmospheric pressure. The system includes a pressure generator and a pressure regulator downstream of the pressure generator. The system monitors a characteristic associated with the pressure regulator, the fluid flow downstream of the pressure regulator to the patient, the fluid flow upstream of the pressure regulator, the fluid flow exhausted from the pressure regulator, or any combination thereof. A controller coupled to the pressure generator controls an operating speed of the pressure generator based on the monitored characteristic(s) so that the output of the pressure generator is only kept as high as needed to provide the desired pressure to the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) fromprovisional U.S. patent application No. 60/499,801 filed Sep. 3, 2003,the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a system and method for providing apressure support therapy to a patient, and, in particular, to a pressuresupport system and method of operating such a system that minimizes theoperating speed of the pressure generating component of the system.

2. Description of the Related Art

It is well known to treat a breathing disorder, such as obstructivesleep apnea (OSA), with a pressure support device, such as a continuouspositive airway pressure (CPAP) device. A CPAP device delivers a flow offluid to the airway of the patient throughout the patient's breathingcycle in order to “splint” the airway, thereby preventing its collapseduring sleep. The term “fluid” as used herein refers to any gas,including a gas mixture or a gas with particles, such as an aerosolmedication, suspended therein. Most commonly, the fluid delivered to apatient by a pressure support system is pressured air. An example ofsuch a CPAP device is the REMstar® and Solo® family of CPAP devicesmanufactured by Respironics, Inc. of Pittsburgh, Pa.

It is also known to provide a bi-level positive pressure therapy inwhich the pressure of fluid delivered to the patient's airway varies oris synchronized with the patient's breathing cycle to maximize thetherapeutic effect and comfort to the patient. An example of a pressuresupport device that provides “bi-level” pressure support, in which alower pressure is delivered to that patient during the patient'sexpiratory phase than during the inspiratory phase, is the BiPAP® familyof devices manufactured and distributed by Respironics, Inc. ofPittsburgh, Pa. Such a bi-level mode of pressure support is taught, forexample, in U.S. Pat. Nos. 5,148,802 to Sanders et al., 5,313,937 toZdrojkowski et al., 5,433,193 to Sanders et al., 5,632,269 toZdrojkowski et al., 5,803,065 to Zdrojkowski et al., 6,029,664 toZdrojkowski et al., 6,305,374 to Zdrojkowski et al., and 6,539,940 toZdrojkowski et al., the contents of each of which are incorporated byreference into the present invention.

It is further known to provide an auto-titration positive pressuretherapy in which the pressure provided to the patient changes based onthe detected conditions of the patient, such as whether the patient issnoring or experiencing an apnea, hypopnea, cheynes-stokes respiration,or upper airway resistance. An exemplary auto-titration pressure supportmode is taught, for example, in U.S. Pat. Nos. 5,203,343; 5,458,137 and6,085,747 all to Axe et al., the contents of which are incorporatedherein by reference. An example of a device that adjusts the pressuredelivered to the patient based on whether or not the patient is snoringis the Virtuoso® CPAP family of devices manufactured and distributed byRespironics, Inc. An example of a pressure support device that activelytests the patient's airway to determine whether obstruction, complete orpartial, could occur and adjusts the pressure output to avoid thisresult is the Tranquility® Auto CPAP device and REMStar Auto CPAPdevice, also manufactured and distributed by Respironics, Inc. Thisauto-titration pressure support mode is taught in U.S. Pat. Nos.5,645,053 and 6,286,508 6,550,478 all to Remmers et al., the content ofwhich is also incorporated herein by reference.

Other pressure support systems that offer other modes of providingpositive pressure to the patient are also known. For example, aproportional assist ventilation (PAV®) mode of pressure support providesa positive pressure therapy in which the pressure of gas delivered tothe patient varies with the patient's breathing effort to increase thecomfort to the patient. U.S. Pat. Nos. 5,044,362 and 5,107,830 both toYounes, the contents of which are incorporated herein by reference,teach a pressure support device capable of operating in a PAV mode.Proportional positive airway pressure (PPAP) devices deliver breathinggas to the patient based on the flow generated by the patient. U.S. Pat.Nos. 5,535,738; 5,794,615; and 6,105,575 all to Estes et al., thecontents of which are incorporated herein by reference, teach a pressuresupport device capable of operating in a PPAP mode. In the PAV and PPAPpressure support systems, the percent of assistance provided by the unitis at least one of the operating features of the pressure support devicethat is set after the device has been prescribed for use by a patient.

A typical conventional pressure support system 10 is shown in FIG. 1.Such a system includes a pressure generator 12 that receives a supply ofgas from a gas source, such as ambient atmosphere, as indicated by arrowA, and creates a flow of breathing gas, as indicated by arrows B, havinga pressure greater than the ambient atmospheric pressure. Pressuregenerator 12 typically includes a motor driving a blower, which is animpeller within a housing, for placing the gas from the gas source underpressure relative to ambient atmosphere. A valve 13 downstream ofpressure generator 12 bleeds off excess pressure/flow from the patientcircuit, as indicated by arrow C, by communicating a portion of the gasat the output of pressure generator 12 to ambient atmosphere. Otherconventional exhaust valves divert all or some of the exhaust gas backto the inlet of the pressure generator.

A patient circuit 14, which is typically a flexible conduit, deliversthe elevated pressure breathing gas to the airway of a patient 15.Typically, the patient circuit is a single limb conduit or lumen havingone end coupled to the pressure generator and a patient interface device16 coupled to the other end. Patient interface device 16 connectspatient circuit 14 with the airway of patient 15 so that the elevatedpressure gas flow is delivered to the patient's airway. Examples ofpatient interface devices include a nasal mask, nasal and oral mask,full face mask, nasal cannula, oral mouthpiece, tracheal tube,endotracheal tube, or hood. A single limb patient circuit shown in FIG.1 includes an exhalation port 18, also referred to as an exhalationvent, exhaust port, or exhaust vent, to allow gas, such as expired gasfrom the patient, to exhaust to atmosphere, as indicated by arrow C.Generally, exhaust vent 18 is located in patient circuit 14 near patientinterface device 16 or in the patient interface device itself.

More sophisticated pressure support devices include a flow sensor 20,pressure sensor 22, or both to monitor the flow and/or pressure of gasin patient circuit 14. The flow information can be used to determine thevolume of gas passing through patient circuit 14. The information fromflow sensor 20 and/or pressure sensor 22 is provided to a controller 24,which uses it for any conventional purpose, for example, to control thepressure or flow of gas provided to the patient, monitor the conditionof the patient, monitor the usage of the pressure support device(patient compliance), or any combination thereof. An input/output device26 communicates with controller 24 to provide data, commands, and otherinformation between a user or other entity and the controller.

In a conventional CPAP system, the pressure output by the blower varieswith the rate of flow in the patient circuit, assuming that the bloweroperates at a constant speed. For example, at a certain operating speed,the pressure in the patient circuit or patient interface decreases asthe flow of gas in the patient circuit or at the patient interfaceincreases. This occurs, for example, as the patient breathes into thepatient circuit. For this reason, a conventional CPAP pressure supportsystem typically uses valve 13 to regulate the pressure delivered to thepatient by means of controller 24.

Devices that provide a bi-level positive pressure therapy, anauto-titrating pressure level, PAV, PPAP, or any other mode of pressuresupport where the pressure delivered to the patient's airway varies,also include a pressure/flow control system to vary the pressuredelivered to the patient in accordance with the pressure support mode. Atypical pressure/flow control system used in such devices includes thepressure/generator and valve combination shown in FIG. 1.

Pressure support systems that include the pressure/generator and valvecombination typically operate the blower at a substantially constantspeed that is sufficient to deliver a pressure in excess of the selectedpressure to be delivered to the patient. That is, the pressure output bythe blower, which is referred to as the “deadhead pressure”, must behigh enough above the selected pressure, or range of pressure, todeliver the prescribed pressure to the patient's airway. Moreover,because it is not known at what air density any given pressure supportsystem will be operated, each system is typically preprogrammed duringmanufacture to operate at an output pressure that is sufficient toaccount for the lowest air density in which the unit will be operated,such as at a high elevation, a lower barometric pressure, hightemperature, or a high humidity.

It can be appreciated that operating the pressure generator tocompensate for the worst case scenario requires outputting much morepressure than needed from the pressure generator. Moreover, bleeding offexcess pressure or flow from the patient circuit via the exhaust valveis relatively noisy, especially in light of the fact that pressuregenerator is operating at a high speed, which itself adds to the machinenoise.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apressure support system that overcomes the shortcomings of conventionalsystems. This object is achieved according to one embodiment of thepresent invention by providing a pressure generator system that includesa pressure generator that generates a flow of fluid at a pressure aboveatmospheric pressure. A conduit is coupled to the output of the pressuregenerator and a pressure sensor is coupled to the conduit at a firstlocation. A valve is operatively coupled to the conduit between thepressure sensor and the end of the conduit. A controller controlsactuation of the valve based on a set pressure to be delivered to apatient and controls the operating speed of the pressure generator basedon an output of the pressure sensor and the set pressure to ensure thatthe output pressure is maintained at a minimum and yet provides thedesired output pressure to the patient.

It is yet another object of the present invention to provide a method ofproviding pressure support to a patient that does not suffer from thedisadvantages associated with conventional pressure support techniques.This object is achieved by providing a method that includes (1)generating a flow of fluid at a pressure above atmospheric pressure viaa pressure generator for delivery to a patient, (2) controlling apressure of the flow of fluid delivered to the patient via a pressurecontrol element so that a pressure delivered to an airway of a patientcorresponds to a set pressure, and (3) controlling an operating speed ofthe pressure generator to provide a pressure at an inlet of the pressurecontrol element at a level sufficient to deliver the set pressure. Thismethod ensures that the pressure provided by the pressure generator tothe pressure control element is maintained at a level that is only highenough to satisfy the pressure support therapy, thereby reducing thewasteful and noisy exhausting of gas from the system.

These and other objects, features and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention. As usedin the specification and in the claims, the singular form of “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional pressure support system;

FIG. 2 is a schematic diagram of a pressure support system according tothe principles of the present invention;

FIGS. 3A and 3B are exemplary pressure and flow graphs illustrating theoperation of the system of FIG. 2;

FIGS. 4A–5B are further exemplary pressure and flow graphs illustratingthe operation of the system of FIG. 2; and

FIG. 6 is a schematic illustration of a feedback control techniqueaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

FIG. 2 schematically illustrates an exemplary embodiment of a pressuresupport system 50 according to the principles of the present invention.The features of the present invention in common with those ofconventional pressure support systems, as shown, for example, in FIG. 1,are delineated with like reference numerals.

Pressure support system 50 employs valve 13 to regulate the pressureoutput from pressure generator 12 so that the desired pressure level isdelivered to the patient. It should be noted that this pressure levelcan be constant, as in the case of a CPAP system or varying, as in thecase of bi-level, PAV, PPAP, or auto-titration pressure support system.Valve 13 acts as a 3-way pressure regulator that pneumatically connectsthe patient's airway in proportional amounts between the outlet pressureor the pressure generator and atmospheric conditions. The pressure to bedelivered to the patient is referred to as the set pressure “P_(SET)”.

In an ideal pressure support system, the pressure at the outlet of valve13 always corresponds to P_(SET) (assuming that the pressure generatoris operating at a sufficient speed to provide this pressure, and bearingin mind any pressure drop that occurs from the outlet of the valve tothe airway of the patient). For example, in a simple CPAP system, thepressure to be delivered to the patient P_(SET) is the CPAP level, whichis typically input into the device via an input interface 26 once thepatient has be prescribed a pressure level. The same is true for asimple bi-level system, where P_(SET) corresponds to the IPAP and EPAPsettings input into the system.

In more complicated pressure support systems, such as PAV, PPAP, orauto-titration pressure support system, as well as CPAP and bi-levelsystems with a pressure ramp function or other pressure control, thepressure to be delivered to the patient P_(SET) is determined, at leastin part, by calculations made within controller 24. Thus, P_(SET) can bea fixed value (CPAP, IPAP, EPAP) or it can be a determined valued basedon any one of a variety of inputs, as is the case with a PAV, PPAP, orauto-titration pressure support system.

The pressure output to the patient varies with many parameters,including, but not limited to, the rate of flow in the patient circuit.Therefore, it is difficult, if not impossible, under normal operatingconditions, to achieve a pressure support system, where the outputpressure from the valve is equal to P_(SET) without directly measuringthe outlet pressure of the valve and using this measured pressure in afeedback fashion. For this reason, a typical pressure support systemincludes pressure sensor 22 at the outlet of valve 13 to monitor thepressure P₂ at the outlet of the valve. The output of pressure sensor 22is provided to controller 24, which controls valve 13 so that thepressure at the outlet of the valve P₂ equals P_(SET). Using thisfeedback control technique, controller 24 continuously adjusts valve 13so that P₂ equals P_(SET). Thus, for present purposes, in which afeedback control is used, the pressure measured by pressure sensor 22(P₂) and the pressure to be delivered to the patient P_(SET) areinterchangeable.

Unlike a conventional pressure support system, one embodiment of thepresent invention provides a first pressure sensor 30 that measures theinlet pressure P₁ to valve 13, which is also the output pressure ofpressure generator 12. The output of pressure sensor 30 is provided tocontroller 24.

Controller 24 regulates the operating speed of pressure generator 12based on the output of pressure sensor 30 and the output of pressuresensor 22 (P₂) or based on the pressure to be delivered to the patient(P_(SET)). More specifically, controller 24 adjusts the operating speed,and hence the output pressure P₁, of pressure generator 12 to providethe lowest possible “overhead pressure”, while still meeting thepressure therapy needs of the patient, i.e., while still ensuring thatthe patient receives a pressure at a level that corresponds to P_(SET).The “overhead pressure” is the pressure that must be delivered to thevalve in order for the valve to provide the desired or prescribedpressure level to the patient's airway in accordance with his or herpressure support therapy. In practice, the “overhead pressure” P_(OH) isthe difference between the pressure measured by the pressure sensor 30and pressure sensor 22, so that P_(OH)=P₁−P_(SET). Because thecontroller regulates valve 13 so that the pressure at the output of thevalve P₂ corresponds to P_(SET), the overhead pressure can also bedescribed as P_(OH)=P₁−P₂.

Conceptually, controller 24 is performing two pressure controloperations:

(1) one based on the pressure to be delivered to the patient P_(SET)(which can be CPAP, bi-level, auto-titrating, PPAP, PAV, or any otherpressure waveform); and (2) one based on the overhead pressure P_(OH)needed to ensure that the minimum overhead pressure is provided to thevalve, while the pressure support system still provides the pressure tothe airway of the patient at a level that is sufficient to meet his orher prescribed or therapeutic need. It should be noted that these twocontrol operations can be carried out independently using separateprocessing elements or carried out by a common processing elementprogrammed to perform both control operations.

FIGS. 3A–4D are graphs illustrating the pressure control process usingpressure sensors 30 and 22 and controller 24 according to the principlesof the present invention. FIGS. 3A and 3B illustrate the following: atypical pressure curve 32 for pressure P₁ measured by pressure sensor30, a typical pressure curve 34 for pressure P_(SET) (which, as notedabove, corresponds to pressure P₂ measured by pressure sensor 22), and atypical flow waveform 36 measured by flow sensor 20 (if used). In thesefigures, a bi-level pressure support therapy is being provided by thepressure support system.

It can be appreciated from reviewing FIG. 3A, that the overhead pressureP_(OH), is relatively large during the expiratory phase of the breathingcycle, as indicated generally at 33, and is relatively small during theinspiratory phase, as indicated at 35. The present invention seeks toensure that a sufficient pressure is provided to the inlet of the valveso that the pressure P_(SET) at the outlet of the valve can bemaintained, thus ensuring that the patient receives their therapeuticpressure while maximizing the operating efficiency of the pressuresupport system. Therefore, it is only important that the pressure P₁ atthe inlet of the valve during the inspiratory phase be considered inorder to achieve this function. For this reason, in an exemplaryembodiment, the controller monitors the overhead pressure P_(OH)(P₁−P_(SET)) to detect the minimum overhead pressure P_(OH)(min), whichoccurs only during the inspiratory phase 35. Detecting the minimumoverhead pressure can be accomplished using any conventional detectiontechnique. It can be appreciated from FIG. 3A, that the overheadpressure is at a minimum during the inspiratory phase 35 of thepatient's respiratory cycle, when the pressure support system controlsthe pressure delivered to the airway of the patient to the IPAP level.See P_(OH)(min1) and P_(OH)(min2) in FIG. 3A.

This minimum overhead pressure P_(OH)(min) is preferably compared to atarget overhead pressure P_(OH)(target). For example, the presentinvention contemplates providing a target overhead pressureP_(OH)(target) of 6 cmH₂O. If the minimum overhead pressure P_(OH)(min)is outside an error band from the target overhead pressureP_(OH)(target), controller 24 adjusts the operating speed of pressuregenerator 12 in a feedback fashion to bring the minimum overheadpressure P_(OH)(min) back to the target overhead pressureP_(OH)(target). In an exemplary embodiment of the present invention, theerror band around the target overhead pressure P_(OH)(target) is ±0.5cmH₂O.

It is to be understood, that this specific target overhead pressureP_(OH)(target) and the range for the error band can be other values.Moreover, the target overhead pressure P_(OH)(target) and the range forthe error band can be adjusted either manually or automatically toachieve the desired range of control for the pressure support system. Itis to be further understood that the error band can be eliminatedentirely.

FIGS. 4A and 4B illustrate a situation where the minimum overheadpressure P_(OH)(min) has fallen outside the error band for the targetoverhead pressure P_(OH)(target), as generally indicated at 40, during afirst inspiratory phase (I₁). This can occur, for example, if the airdensity in which the pressure support system is operating has changed,for example, if the unit is moved to a higher elevation or if theambient barometric pressure has dropped. This can also occur if theleakage of gas from the pressurized system is increased, e.g., thepatient has moved causing a leak between the mask seal (cushion) and thesurface of the patient, or if the patient has taken a relatively largeinspiratory breath, such as a large sigh. When this occurs, theefficiency of the pressure generator is reduced so that the outputpressure P₁ is not as high as it was at the lower elevation, higherbarometric pressure, less system leak, and/or smaller inspiratorybreath.

When the controller determines that the minimum overhead pressureP_(OH)(min) does not correspond to the target overhead pressureP_(OH)(target), controller 24 begins increasing the motor speed, asindicated generally at 42. The increase in motor speed translates intoan increase in pressure level P₁ beginning at 42. During the nextinspiratory phase (I₂), the minimum overhead pressure P_(OH)(min)detected during that phase is again compared to the target overheadpressure P_(OH)(target). As generally indicated at 44, the minimumoverhead pressure P_(OH)(min) is still less than the target overheadpressure P_(OH)(target). Thus, the motor speed is again increased (orcontinues to be increased) as indicated at 46. During the thirdinspiratory phase (I₃), the minimum overhead pressure P_(OH)(min) is nolonger less than the target overhead pressure P_(OH)(target), asindicated at 48. Thus, the motor speed is held constant, as indicated at50. Of course, if the minimum overhead pressure P_(OH)(min) shouldincrease above the target overhead pressure P_(OH)(target), the motorspeed is decreased. An example of this is shown in FIG. 5A. Again, it isto be understood, that if an error band is used with the target overheadpressure P_(OH)(target), the minimum overhead pressure P_(OH)(min) mustfall outside this error band before a motor speed adjustment is made.

The present invention contemplates that the rate at which the speed ofthe motor in the pressure generator is changed can be fixed or variable.For example, the rate at which the motor speed is increased (ordecreased) can be based on the amount of difference between the minimumoverhead pressure P_(OH)(min) and the target overhead pressureP_(OH)(target). Thus, the further the minimum overhead pressureP_(OH)(min) is below the target overhead pressure P_(OH)(target), themore the speed is increased. When using a fixed rate of change in motorspeed, the rate of change can be set to be relatively fast or relativelyslow, depending on the tradeoffs the user is willing to accept. Arelatively fast rate of change allows the system to quickly correct thepressure overhead error. However, it is believed that corrections inpressure made too rapidly can be detected by the patient using thesystem, thereby jeopardizing their comfort with the system. In apreferred embodiment of the present invention, the rate of change is setsuch that it takes approximately 10 breaths for the system to transitionfrom a P_(OH) of 0 cmH₂O to a P_(OH) of 6 cmH₂O.

In the illustrated embodiment, the motor speed of the pressure generatoris adjusted (increased or decreased) at the start of the expiratoryphase following the inspiratory phase in which the need for anadjustment was detected. This is so because in this embodiment, thecontroller first searches for the minimum overhead pressure. P_(OH)(min)during the inspiratory phase and uses this value compared to thethreshold to determine whether or not to adjust the output pressure P₁.It is to be understood, however, that other techniques of controllingthe motor speed, and hence, the output pressure P₁ are contemplated bythe present invention. For example, the present invention contemplatescontinuously monitoring the overhead pressure (P₁−P_(SET)) during theinspiratory phase and increasing the operating speed immediately whenthe overhead pressure falls below the minimum pressure thresholdP_(min).

In the embodiment illustrated in FIG. 4A, during inspiratory phase I₁,the output pressure P₁ is actually below the set pressure P_(SET) to bedelivered to the patient. As noted above, the set pressure P_(SET)corresponds to the pressure that the system is supposed to be deliveringto the patient, such as the CPAP pressure or the IPAP pressure. Thismeans, that even if valve 13 is fully closed so that no gas flows toatmosphere and all gas generated by the pressure generator iscommunicated by the valve to the patient circuit, the pressure of gas P₂being delivered to the patient is below that which he or she is supposedto be receiving. The present invention contemplates two techniques fordealing with this situation. It should be noted that regardless of thetechnique used, the pressure delivered to the patient P₂ will not meetthe set pressure until the motor speed (output pressure P₁ from thepressure generator) is increased.

According to a first technique, the pressure to be delivered to thepatient P_(SET) is allowed to be equal to the pressure delivered to thevalve P₁ (less whatever small pressure drop may occur in the valve). Inshort, P₂ or P_(SET)=P₁. The disadvantage of this technique is that itrequires, by definition, that valve 13 move to its fully actuatedposition so that no gas flows to atmosphere and all gas generated by thepressure generator is communicated by the valve to the patient circuit.This is typically referred to as allowing the valve to “hit therail”—where the physical constraints of the valve prevent it fromopening a path from the pressure generator to the patient circuit anywider. To ensure that valve 13 operates correctly over a long period oftime, it is preferable to avoid allowing the moveable elements of thevalve to “hit the rail”.

Thus, a second technique, which is shown in FIG. 4A, has been developedfor use with the control system of the present invention. According tothis technique, when the input pressure to the valve P₁ falls below thepressure to be delivered to the patient P_(SET), the pressure to bedelivered to the patient P_(SET) is recalculated as follows:P_(SET)=P₁−k, where k is a constant. Constant k is selected to preventthe valve from being moved to its fully actuated position. Constant k isillustrated graphically in FIG. 4A as a gap 52 between P₁ and P₂ duringinspiratory phase I₁.

It is to be understood that other techniques for controlling thepressure so that the valve does not “hit the rail” are contemplated bythe present invention. For example, when P₁<P_(SET), P_(SET) can berecalculated as a function of the flow in the patient circuit. In oneembodiment P_(SET)=P₁−f(Q), where Q is the rate of fluid flow in thepatient circuit, which is typically measured by flow sensor 20. Thepresent invention also contemplates monitoring the position of the valvean controlling its actuation so that it does not “hit the rail” byadjusting P_(SET) accordingly to achieve this function.

FIGS. 5A and 5B illustrate a situation where the minimum overheadpressure P_(OH)(min) is within acceptable levels during inspiratoryphases 14 and 15, but has risen above the maximum pressure thresholdP_(max), as generally indicated at 60, during inspiratory phase I₆. Thiscan occur, for example, if a leak from the pressurized system isdecreased or removed, e.g., the patient has adjusted his or her mask tocorrect a leak between the mask seal (cushion) and the surface of thepatient. When this occurs, the head losses in the pressure generatordecrease so that the rotating speed of the motor in the pressuregenerator need not be as high as it was when there was a larger leak

When controller 24 determines that the minimum overhead pressureP_(OH)(min) is above the target overhead pressure P_(OH)(target),controller 24 begins decreasing the motor speed, as indicated generallyat 62. The decrease in motor speed translates into a decrease inpressure level P₁ beginning at 62. During the next inspiratory phase(I₇), the minimum overhead pressure P_(OH)(min) detected during thatphase is again compared to the target overhead pressure P_(OH)(target).During inspiratory phase I₇, the minimum overhead pressure P_(OH)(min)is no longer greater than the target overhead pressure P_(OH)(target).Thus, the motor speed is held constant, as indicated at 64.

FIG. 5B illustrates a patient flow 66, which is the flow of gas into andout of the patient, and a total flow 68, which corresponds to the flowindicated by arrow B in FIG. 2 and is the sum of all flows due to leaks(intentional and unintentional) and the patient flow. FIG. 5Billustrates the drop in flow due to leak from a first level 70 to asecond, lower level 72 as a result of the leak correction occurring at60. Is should be noted that the patient flow 66 remains unchangeddespite changes in the leak rate from the pressurized system.

It can be appreciated from the above description that the motor speedcontrol of the present invention provides the same function as abarometric pressure sensor without the additional sensor cost whileadditionally compensating for all head losses in the system, whichcannot be accomplished with a barometric pressure sensor. The presentinvention also minimizes the output pressure generated by the pressuregenerator while automatically compensating for pressure losses due toleak, changes in motor efficiency (such as patient breathing in thepatient circuit), and changes in the ambient atmosphere. In addition,audible noise from motor vibration is minimized for a given set ofatmospheric and patient load conditions.

In the illustrated embodiment, separate pressure sensors 22 and 30 areprovided on either side of valve 13. Each pressure sensor 22 and 30 isreferenced to atmosphere. It is to be understood, that the presentinvention also contemplates providing a differential pressure sensorconnected across valve 13 to directly measure the overhead pressure.That is each side of the pressure sensor is connected to the patientcircuit on each respective side of valve 13. An advantage of using adifferential pressure sensor in this configuration is that thedifferential pressure sensor need not be calibrated to atmosphericpressure. In this embodiment, pressure sensor 22 is still needed inorder to ensure that the set pressure, i.e., the desired therapypressure, is delivered to the patient. FIG. 6 schematically illustratesthe feedback control technique for controlling the operating speed ofthe pressure generator based on pressure P₁ measured at the inlet to theexhaust valve and pressure P₂ measured at the outlet to the exhaustvalve.

It should also be noted that flow sensor 20 is optional. Flow sensor 20is typically used in pressure support systems that require leakestimation, such as bi-level and auto-titration systems, and systemsthat control the pressure based at least in part on flow, such as PPAPand PAV systems.

Pressure sensor 22 can be provided anywhere between valve 13 and thepatient's airway. If the pressure sensor is connected to the patientcircuit at a location that is distal from the patient, as shown in FIG.2, conventional techniques to estimate the pressure at the airway of thepatient, for example, by compensating for the pressure drop alongconduit 14, are used.

In the embodiments shown in FIGS. 3A–5B a bi-level mode of pressuresupport is shown. It is to be understood, however, that the pressuresupport system of the present invention can provide any type of pressuresupport. CPAP, bi-level, auto titration, PAV, PPAP, etc., as well as anycombination thereof. In addition, the pressure support system caninclude any conventional devices, such a humidifier, bacterial filters,alarms, compliance monitors, etc., typically used in pressure supportsystems.

In the embodiment described above this minimum overhead pressureP_(OH)(min) is compared to a target overhead pressure P_(OH)(target),with or without an error band. It is to be understood, however, thepresent invention contemplates other techniques for controlling theoverhead pressure. For example, in another embodiment of the presentinvention, the minimum overhead pressure P_(OH)(min) is compared to aminimum overhead pressure threshold P_(OH)(threshold(min)), a maximumthreshold P_(OH)(threshold(max)), or both. So long as the minimumoverhead pressure P_(OH)(min) remains within these thresholds, the motorspeed is not changed. If, however, the minimum overhead pressureP_(OH)(min) falls below the minimum overhead pressure thresholdP_(OH)(threshold(min)), the output of the pressure generator isincreased, and if the minimum overhead pressure P_(OH)(min) rises abovethe maximum overhead pressure threshold P_(OH)(threshold(max)), theoutput of the pressure generator is decreased. Thus, in this embodiment,the minimum overhead pressure threshold P_(OH)(threshold(min)) andmaximum threshold P_(OH)(threshold(max)) define the boundaries for thefluctuations in the minimum overhead pressure P_(OH)(min).

In the invention described immediately above, the minimum overheadpressure is compared to a maximum threshold level P_(max), and a minimumpressure threshold P_(min) and the output of the pressure generator isadjusted accordingly. It is to be understood, however, that the presentinvention contemplates comparing the minimum overhead pressure to theminimum pressure threshold P_(min) only. The output of the pressuregenerator (operating speed) is increased if the minimum overheadpressure falls below the minimum pressure threshold P_(min). To allowfor reductions in the output of the pressure generator, the controllercan periodically decrease the output (reduce the operating speed) of thepressure generator by some amount. So long as the minimum overheadpressure remains above the minimum pressure threshold P_(min) thereduction is allowed. This process can be repeated so that the systemeffectively “searches” for the lowest operating speed for the pressuregenerator while maintaining the minimum overhead pressure remains abovethe minimum pressure threshold P_(min).

The converse is also possible. That is, the present inventioncontemplates comparing the minimum overhead pressure to the maximumpressure threshold P_(max) only. The output of the pressure generator(operating speed) is decreased if the minimum overhead pressure is abovethe maximum pressure threshold P_(max). The controller can periodicallyincrease the output (increase the operating speed) of the pressuregenerator by some amount. So long as the minimum overhead pressureremains below the maximum pressure threshold P_(max) the increase isallowed. This process can be repeated so that the system effectively“searches” for the highest operating speed for the pressure generatorwhile maintaining the minimum overhead pressure remains below themaximum pressure threshold P_(max).

In addition to or in place of controlling the operating speed of thepressure generator to ensure that an acceptable overhead pressure ismaintained, the present invention also contemplates controlling theoperating speed of the pressure generator based on other criteria. Thatis, the prevent invention contemplates that the pressure support systemmonitor a characteristic associated with the pressure regulator, i.e.,the valve, the fluid flow downstream of the pressure regulator, thefluid flow upstream of the pressure regulator, the fluid flow exhaustedfrom the pressure regulator, or any combination thereof. The controllerthen controls an operating speed of the pressure generator based on themonitored characteristic(s) so that the output of the pressure generatoris only kept as high as needed to provide the desired pressure to thepatient.

In one embodiment of the present invention, the preset inventioncontemplates controlling the operating speed of the blower based on theexhaust flow from the patient circuit via valve 13. For example, a flowsensor can be provided to detect the flow of exhaust gas C (see FIG. 2)and provide a feedback signal to controller 24. The controller adjuststhe operating speed to the pressure generator to minimize the exhaust orwasted flow dumped from the patient circuit.

In another embodiment of the present invention, the shape of thepressure waveform at the outlet of the valve, as the patient, or at bothlocations are monitored. The operating speed of the pressure generatoris adjusted minimize any error or difference between the desired therapypressure waveform and the measured therapy pressure waveform. Forexample, the shape of waveform P_(SET) (i.e., P₂) is monitored. As shownin FIG. 4A, if the pressure overhead becomes to small, the waveformshape changes, as is the case during inspiratory cycle I₁. The presentinvention contemplates detecting the deviations of the waveform to bedelivered to the patient from an model waveform or from expected valuesof the waveform, such as peak values, and adjusting the operating speedof the pressure generator to correct for these deviations. Thisembodiment has the advantage in that only one pressure sensor is neededat the outlet of the valve, and no pressure sensor is need at the valveinlet.

Still another embodiment of the present invention contemplates providinga flow sensor on the valve inlet and a flow sensor on the valve outlet.The operating speed of the pressure generator is adjusted so that theflow measured by both flow sensors during the inspiratory phase, i.e.,during the IPAP therapy level in a bi-level system, is nearly equal.

Yet another embodiment of the present invention contemplates monitoringthe patients flow waveform and adjusting the operating speed of thepressure generator to provide the maximum inspired tidal volume for agiven pressure therapy.

Another embodiment of the present invention contemplates monitoring thepatient's flow and adjusting the operating speed of the pressuregenerator to provide to provide the maximum peak inspired flow for agiven pressure therapy.

A still further embodiment of the present invention contemplatesmonitoring the movement of valve 13. The operating speed of the pressuregenerator is adjusted to insure that the valve is traversing the entirestroke, i.e., is operating over its entire range of movement.

A further embodiment of the present invention that is related to theimmediately preceding embodiment contemplates monitoring the valvecurrent, i.e., the energy provided to the valve 13, or othercharacteristics associated with the movement of the valve. The operatingspeed of the pressure generator is adjusted to insure that the valvecurrent (or other movement characteristic) is maximized.

Yet another embodiment of the present invention contemplates monitoringa control parameter, e.g. the integrator term in a PID, for the valvecontroller. The operating speed of the pressure generator is adjusted tominimize the integral term.

Still another embodiment of the present invention, contemplatesproviding a microphone or (accelerometer) on the pressure generator andadjusting the operating speed of the pressure generator to minimize thesound pressure level or amplitude of vibration produced by the pressuregenerator while still maintaining the outlet pressure therapy P_(SET).

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims.

1. A pressure support system comprising: a pressure generator adapted togenerate a flow or fluid at a pressure above atmospheric pressure; aconduit having a first end coupled to the output of the pressuregenerator and a second end; a first pressure sensor operatively coupledto the conduit at a first location; a valve operatively coupled to theconduit between the first pressure sensor and the second end of theconduit, wherein the valve is adapted to control a pressure of fluiddelivered to the second end of the conduit; a second pressure sensorcoupled to the conduit at a second location between the valve and thesecond end of the conduit; and a controller operatively coupled to thepressure generator, the valve, and the first pressure sensor, whereinthe controller controls actuation of the valve based on a set pressureto be delivered to a patient, which is a pressure measured by the secondpressure sensor, and wherein the control ter controls an operating speedof the pressure generator based on an output of the first pressuresensor and the set pressure.
 2. The system of claim 1, wherein the firstpressure sensor and the second pressure sensor are defined by adifferential pressure sensor having a first pressure portion thatcorresponds to a pressure in the conduit at the first location and asecond pressure portion that corresponds to a pressure in the conduit atthe second location, wherein the differential pressure sensor monitors apressure between the first pressure portion and the second pressureportion as the pressure difference.
 3. The system of claim 1, furthercomprising an input device operative coupled to the controller, whereinthe set pressure is determined based on an input from the input device.4. The system of claim 1, wherein the pressure generator comprises animpeller and a motor operatively coupled to the impeller such thatoperation of the motor rotates the impeller, and wherein the controllercontrols an operating speed of the motor.
 5. The system of claim 1,further comprising a patient interface device coupled to the second endof the conduit.
 6. The system of claim 1, wherein the set pressureincludes an inspiratory positive airway pressure (IPAP) during at leasta portion of an inspiratory phase of a respiratory cycle and anexpiratory positive airway pressure (EPAP) during at least a portion ofan expiratory phase of the respiratory cycle, and wherein the IPAP levelis greater than the EPAP level.
 7. The system of claim 2, wherein theset pressure includes an inspiratory positive airway pressure (IPAP)during at least a portion of an inspiratory phase of a respiratory cycleand an expiratory positive airway pressure (EPAP) during at least aportion of an expiratory phase of the respiratory cycle, and wherein theIPAP level is greater than the EPAP level.
 8. A pressure support systemcomprising: a pressure generator adapted to generate a flow of fluid ata pressure above atmospheric pressure; a conduit having a first endcoupled to the output of the pressure generator and a second end; afirst pressure sensor operatively coupled to the conduit at a firstlocation; a valve operatively coupled to the conduit, wherein the valveis adapted to control a pressure of fluid delivered to the second end ofthe conduit; and a controller operatively coupled to the pressuregenerator, the valve, and the first pressure sensor, wherein thecontroller controls actuation of the valve based on a set pressure to bedelivered to a patient, wherein the controller controls an operatingspeed of the pressure generator based on an output of the first pressuresensor and the set pressure, and wherein the controller monitors anoverhead pressure as a difference between a pressure measured by thefirst pressure sensor and the set pressure, and controls the pressuregenerator based on the overhead pressure.
 9. The system of claim 8,wherein the controller compares the overhead pressure to a minimumpressure threshold P_(min) and increases an operating speed of thepressure generator responsive to the overhead pressure being less thanthe minimum pressure threshold P_(min).
 10. The system of claim 8,wherein the controller determines a minimum overhead pressure during aninspiratory phase of such a patient, compares the minimum overheadpressure to a minimum pressure threshold P_(min) and increases anoperating speed of the pressure generator responsive to the overheadpressure being less than the minimum pressure threshold P_(min).
 11. Thesystem of claim 8, wherein the controller determines a minimum overheadpressure during an inspiratory phase of such a patient, compares theminimum overhead pressure to a maximum pressure threshold P_(max) anddecreases an operating speed of the pressure generator responsive to theminimum overhead pressure being greater than the maximum pressurethreshold P_(max).
 12. The system of claim 8, wherein the controllercompares the overhead pressure to a target overhead pressureP_(OH)(target) and adjusts an operating speed of the pressure generatorto cause the overhead pressure to correspond to the target overheadpressure P_(OH)(target).
 13. The system of claim 8, her comprising asecond pressure sensor coupled to the conduit at a second location, andwherein the set pressure is a pressure measured by the second pressuresensor.
 14. The system of claim 8, further comprising an input deviceoperative coupled to the controller, wherein the set pressure isdetermined based on an input from the input device.
 15. A pressuresupport system comprising: a pressure generator adapted to generate aflow of fluid at a pressure above atmospheric pressure; a conduit havinga first end coupled to the output of the pressure generator and a secondend; a valve operatively coupled to the conduit and adapted to control apressure of the flow of fluid delivered to such a patient; firstcontrolling means operatively coupled to the valve for controlling thevalve to deliver so that a pressure delivered to an airway of a patientcorresponds to a set pressure; and second controlling means operativelycoupled to the pressure generator for controlling an operating speed ofthe pressure generator to provide a pressure at an inlet of the valve ata level sufficient to deliver the set pressure.
 16. The system of claim15, wherein the first controlling means and the second controlling meansare defined by a common processing element.
 17. The system of claim 15,further comprising pressure monitoring means for determining a pressureat an inlet of the valve, and wherein the second controlling meanscontrols the operating speed based on an output of the pressuremonitoring means.
 18. The system of claim 15, further comprisingpressure monitoring means for determining a pressure difference acrossthe valve, and wherein the second controlling means controls anoperating speed of the pressure generating means based on the pressuredifference.
 19. The system of claim 15, further comprising a patientinterface device coupled to the second end of the conduit.
 20. Thesystem of claim 15, wherein the first controlling means provides aninspiratory positive airway pressure (IPAP) during at least a portion ofan inspiratory phase of a respiratory cycle and an expiratory positiveairway pressure (EPAP) during at least a portion of an expiratory phaseof the respiratory cycle, and wherein the IPAP level is greater than theEPAP level.
 21. The system of claim 15, wherein the second controllingmeans determines an overhead pressure across the valve and compares theoverhead pressure to a target overhead pressure P_(OH)(target), andadjusts the operating speed of the pressure generator to cause theoverhead pressure to correspond to the target overhead pressureP_(OH)(target).
 22. The system of claim 15, wherein the secondcontrolling means determines a minimum overhead pressure during aninspiratory phase of such a patient compares the minimum overheadpressure to a maximum pressure threshold P_(max) and decreases anoperating speed of the pressure generating means responsive to theminimum overhead pressure being greater than the maximum pressurethreshold P_(max).
 23. The system of claim 15, wherein the secondcontrolling means determines a minimum overhead pressure during aninspiratory phase of such a patient, compares the minimum overheadpressure to a minimum pressure threshold P_(min) and increases anoperating speed of the pressure generating means responsive to theminimum overhead pressure being less than the minimum pressure thresholdP_(min).
 24. A method of providing pressure support to a patientcomprising: generating a flow of fluid at a pressure above atmosphericpressure via a pressure generator; providing the flow of fluid to apatient; controlling a pressure of the flow of fluid delivered to such apatient via a pressure control element so that a pressure delivered toan airway of a patient corresponds to a set pressure; and controlling anoperating speed of the pressure generator to provide a pressure at aninlet of the pressure control element at a level sufficient to deliverthe set pressure.
 25. The method of claim 24, further monitoring apressure at an inlet of the pressure control element, and whereincontrolling the operating speed is done based the monitored pressure.26. The method of claim 24, further monitoring a pressure differenceacross the pressure control element, and wherein controlling theoperating speed is done based on the pressure difference.
 27. The methodof claim 26, wherein controlling the operating speed includes comparingthe pressure difference to a threshold level and controlling thepressure generator based on the comparison.
 28. The method of claim 24,wherein controlling a pressure of the flow of fluid delivered to such apatient via a pressure control element includes providing the flow offluid at an inspiratory positive airway pressure (IPAP) during at leasta portion of an inspiratory phase of a respiratory cycle and at anexpiratory positive airway pressure (EPAP) during at least a portion ofan expiratory phase of the respiratory cycle, and wherein the IPAP levelis greater than the EPAP level.
 29. The method of claim 24, whereincontrolling the operating speed of the pressure generator includesdetermining an overhead pressure across the pressure control element,comparing the overhead pressure to a target overhead pressureP_(OH)(target), and adjusts the operating speed of the pressuregenerator to cause the overhead pressure to correspond to the targetoverhead pressure P_(OH)(target).
 30. The method of claim 24, whereincontrolling the operating speed of the pressure generator includesdetermining a minimum overhead pressure during an inspiratory phase ofsuch a patient, comparing the minimum overhead pressure to a maximumpressure threshold P_(max), and decreasing the operating speed of thepressure generator responsive to the minimum overhead pressure beinggreater than the maximum pressure threshold P_(max).
 31. The method ofclaim 24, wherein controlling the operating speed of the pressuregenerator includes determining a minimum overhead pressure during aninspiratory phase of such a patient, comparing the minimum overheadpressure to a minimum pressure threshold P_(min), and increasing theoperating speed of the pressure generator responsive to the minimumoverhead pressure being less than the minimum pressure thresholdP_(min).